1. Field of the Invention
The present invention relates to a technology for highly accurately discriminating a liquid substance which is supplied to a nitrogen oxide reduction catalytic converter disposed in an engine exhaust system of a moving vehicle, by utilizing a concentration sensor which detects the concentration of a liquid reducing agent based on a heat transfer characteristic between two positions spaced apart from one another.
2. Description of the Related Art
As a catalytic purification system for removing nitrogen oxides (NOx) contained in the exhaust emission of an engine, there has been proposed an exhaust emission purifying apparatus disclosed in Japanese Unexamined Patent Publication No. 2000-27627. In the exhaust emission purifying apparatus, a NOx reduction catalytic converter is disposed in an engine exhaust system, and a reducing agent is injection-supplied to the exhaust upstream of the NOx reduction catalytic converter, so that NOx in the exhaust emission and the reducing agent are subjected to the catalytic-reduction reaction, to thereby purify NOx into harmless components The reducing agent is stored in a storage tank in a liquid state at an ordinary temperature, and a necessary amount thereof corresponding to engine operating conditions is injection-supplied from an injection nozzle. Further, for the reduction reaction, ammonia having the good reactivity to NOx is used, and as the reducing agent, the urea aqueous solution, the ammonia aqueous solution or another type aqueous reducing agent solution (to be referred to as a liquid reducing agent hereunder), which hydrolyze due to heat of the exhaust emission and water vapor to easily generate ammonia, is used.
However, according to the above-described conventional exhaust emission purifying apparatus, when the concentration of the liquid reducing agent is changed due to any factor, if a driver continues operating of the engine without taking notice of this change, there is a possibility that the NOx reduction efficiency by the NOx reduction catalytic converter is lowered, and therefore, the required NOx purification performance cannot be achieved. In particular, continually operating the engine, irrespective of improper mixing ratio between the reducing agent and the solvent in the liquid reducing agent, despite of contamination of the liquid reducing agent by another type of aqueous solution or the solvent, or the lack of the residue of the liquid reducing agent or the like, raises a possibility of a large amount of discharge of NOx from the engine.
Therefore, it is considered that a concentration sensor which detects the concentration of the liquid reducing agent based on heat transfer characteristics between two positions spaced apart from each other is disposed in the storage tank. However, if such a concentration sensor is mounted on a moving vehicle such as an automobile, the following problems are caused. Namely, during the driving of the moving vehicle, since a vehicle body is continually subjected to vibration due to the irregularity of road surfaces, the convection can be generated in the liquid reducing agent in the storage tank. If the convection is generated in the liquid reducing agent, the heat transfer characteristics using the liquid reducing agent as a heat transfer medium is changed, and therefore, the detection accuracy of the concentration of the liquid reducing agent is significantly lowered. On the other hand, since the NOx purification is hardly expected unless the liquid reducing agent is supplied to the NOx reduction catalytic converter, it is necessary to discriminate at least whether the liquid in the storage tank is the water or the liquid reducing agent, or the storage tank is empty.